The present invention relates generally to the art of diagnostic imaging. It finds particular application to magnetic resonance angiography (MRA) techniques, apparatuses, as well as to methods and apparatuses for the digital processing of image data acquired by the same. Although the present invention is illustrated and described herein primarily with reference to magnetic resonance angiography, it will be appreciated that the present invention is also amenable to other imaging modalities and to subjects other than the human body.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field (B0) is set up in an examination region in which a subject being imaged or examined is placed. Via magnetic resonance radio frequency (RF) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped to excite magnetic resonance. The resonance is typically manipulated to induce detectable magnetic resonance echoes from a selected region of the subject. In imaging, the echoes are spatially encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix, commonly known as k-space. By employing inverse Fourier, two-dimensional Fourier, three-dimensional Fourier, or other known transformations, an image representation of the subject is reconstructed from the k-space data.
A number of MR angiography (MRA) techniques have been developed for imaging the vascular system. Time-of-flight (TOF) techniques rely on the time interval between the transverse excitation of spins and the acquisition of the resulting magnetic resonance signal to distinguish between moving and stationary spins. During the time interval, fresh spins move into the region from which the magnetic resonance signal is acquired and excited spins move out of the region. In contrast, the stationary spins remain fixed during the interval between RF excitation and data acquisition, with the result that the magnetic resonance signal produced by stationary spins is substantially different in magnitude from that produced by moving spins. When an image is reconstructed from such magnetic resonance signals, the image pixels which correspond to moving spins are either much brighter or much darker than image pixels corresponding to stationary spins, depending on the sequence. In this manner, the vascular system that transports moving blood is made to appear brighter or darker than the surrounding stationary or slowly moving tissues in the resultant image.
Phase contrast techniques rely on the fact that the phase of the magnetic resonance signal produced by moving spins is different from the phase of magnetic resonance signals produced by stationary or slowly moving spins. Phase contrast methods employ magnetic field gradients during the magnetic resonance pulse sequence which cause the phase of the resulting magnetic resonance signals to be modulated as a function of spin velocity. The phase of the magnetic resonance signals can, therefore, be used to control the contrast, or brightness, of the pixels in the reconstructed image. Since blood is moving relatively fast, the vascular system is made to appear brighter or darker in the resulting image.
Contrast-enhanced magnetic resonance angiography has been employed to enhance the diagnostic capability of magnetic resonance angiography. In contrast-enhanced magnetic resonance angiography, a contrast agent such as gadolinium is injected into the patient prior to the scan. The injection is carefully timed so that the central lines of k-space, which govern image contrast, are acquired during peak arterial enhancement, i.e., at the moment the bolus of contrast agent is flowing through the vasculature of interest.
Irrespective of the method used to acquire the magnetic resonance data, MRI can be used to produce volume image data comprising a three-dimensional (3D) array of voxel intensities. This volume image data can be acquired using a three-dimensional pulse sequence or with two-dimensional (2D) pulse sequences applied to a plurality of adjacent slices. Also, three-dimensional pulse sequences can be applied to a plurality of adjacent subvolumes to achieve coverage of the desired volume of interest.
Maximum intensity projection (MIP) is a common and powerful tool for rendering three-dimensional volume image data sets, and is particularly useful in connection with magnetic resonance angiographic images. Projection images are especially useful for screening vascular morphology and pathological diseases, such as stenosis, atherosclerosis, and aneurysm. In clinical diagnosis, viewing projection images is generally preferable to viewing individual MRI slice images.
An MIP image is essentially a projection of a three-dimensional volume onto a two-dimensional plane from a designated viewing point or along a designated viewing angle or projection angle. In producing an MIP image, a ray is projected from each pixel in the two-dimensional projection image plane through the three-dimensional array of image data points. The value for the data point along the ray that has the maximum intensity value is selected. The value thus selected for each ray is used to control the gray scale of its corresponding pixel in the resultant two-dimensional image. MIP images can be acquired from different viewing angles or viewing positions, providing radiologists with flexibility to study cases. Similarly, minimum intensity projection (MinIP) is used for black blood angiographic applications. In MinIP, the resonance response from flowing blood is minimized by the magnetic resonance sequence such that blood is depicted as black in the resultant image. The lowest voxel intensity along each ray is assigned to the corresponding pixel in the resultant two-dimensional projection image.
In contrast angiography, the bolus of contrast agent moves dynamically through the image area. Also, blood surges and slows cyclically with the cardiac cycle. The MR images freeze this motion as a snap shot in time. In some examinations, multiple images are acquired displaced in time or phase of the cardiac cycle. The region of interest is then displayed in a cinematic display.
Current MIP techniques do not include the time variant effect of blood flow behavior, e.g., affected by cardiac cycle, arrival and departure of contrast substances, etc. Rather, they assume that the acquired blood flow behavior is static. Although a number of algorithms have been developed to increase vessel edge definition, such as reconstruction grid repositioning and various interpolation techniques, such techniques cannot correct inaccuracies due to such time varying effects.
Accordingly, the present invention contemplates a new and improved magnetic resonance angiography apparatus and method wherein the time varying effects are incorporated into the final projection which overcomes the above-referenced problems and others.
In accordance with a first aspect of the present invention, an angiographic imaging method is provided. A plurality of temporally displaced volumetric image representations of a volume of interest is generated, each depicted by a three-dimensional array of voxel values. The plurality of three-dimensional arrays of voxel values is temporally collapsed in accordance with a selected criterion to generate a temporally collapsed three-dimensional array of voxel values, which is projected in a selected direction in accordance with the selected criterion to generate a two-dimensional image representation.
In accordance with another aspect of the present invention, a diagnostic imaging method is provided. A plurality of temporally displaced volumetric image representations of a volume of interest is generated, each depicted by a three-dimensional array of voxel values. The plurality of temporally displaced volumes is then spatially and temporally collapsed in accordance with a selected criterion to generate a spatially and temporally collapsed two-dimensional image representation.
In accordance with another aspect of the present invention, an image processing system is provided. A reconstruction processor and associated memory generates and stores a plurality of volume image representations of a volume of interest corresponding to a plurality of offset times. A time course projection processor and associated memory temporally collapses and stores a plurality of temporally offset image representations into a single temporally collapsed image representation. A spatial projection processor and associated memory generates two-dimensional spatially projected image representations from the volume image representations. A first of the time course and spatial projection processors is connected with the reconstruction processor and a second of the time course and spatial projection processors is connected with the first for generating a two-dimensional temporally collapsed and spatially projected image representation. A display apparatus converts the two-dimensional temporally collapsed and spatially projected image representation into a human-viewable image.
One advantage of the present invention is that it improves accuracy in depicting blood vessel lumen of imaged blood vessels.
Another advantage is that it captures blood flow time variations in vivo.
Another advantage of the present invention is that it compensates for the dynamics of the blood vessel wall, e.g., fluctuating lumen size.
Another advantage of the present invention resides in its ready adaptability to myriad diagnostic imaging acquisition techniques.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.